1. Field of the Invention
The present invention relates to an optical disk apparatus for recording and reproducing information to and from an optical disk and, more particularly, to an optical disk apparatus which can correctly access recording tracks formed on the optical disk regardless of physical distortion of the disk and regardless of an electrical distortion of a tracking system.
2. Description of the Prior Art
In recent years, an optical disk apparatus has been developed for recording and reproducing a large quantity of data. For this purpose, many approaches for a higher recording density are made. For achieving such a high recording density, it is necessary to reduce the inter-recording pit interference, improve the S/N ratio, and compensate the signal quality for the degradation caused by the fluctuations of the optical disk medium and/or the optical disk apparatus. Specifically, such fluctuations mainly relate to the physical distortion of the optical disk and the propensities of the electrical circuit used in the optical disk apparatus.
With reference to FIG. 75, a first case, relating to the physical distortion of the optical disk, is described. Principally, an optical disk D should be kept in a flat disk shape when it is set in a disk motor Md, so that an optical pickup Os can keep its optical axis perpendicular to the recording surface of the disk D during recording and reproducing operations. For scanning the recording tracks, the optical pickup Os moves in a radial direction indicated by an arrow Dr on a line LR in alignment with the radius of the optical disk D.
However, the optical disk D set in the disk motor Md will hang down from its outer circumference due to its weight. In other words, the optical disk D inclines downward in a conical shape instead of the flat plate disk shape. Strictly speaking, the optical disk D also curves both in the radial and circumferential directions. Therefore, as a result of such an inclination of the disk D, the optical pickup Os can not scan the recording tracks with its optical axis Ax across the recording surface of the disk D at a right angle. Furthermore, the crossing angle varies according to the position of the optical pick Os with respect to the optical disk D (the line LR).
Under such conditions, an angle .theta.r formed between the optical axis Ax and the recording surface of the disk D is defined as a "radial tilt angle." An angle .theta.t formed between the optical axis Ax and a line LD tangential to the recording track (or perpendicular to the radius) of the optical disk D is referred to as a "tangential tilt." Generally, the amount of radial tilt .theta.r of the optical disk, such as for a compact disk, is within the range from -0.5.degree. to 0.5.degree..
The second case, relating to the propensities of electric circuits, appears such that the laser beam can not be correctly focused and tracked on the target recording track even when the focus or tracking signal indicates that the laser spot is correctly focused and tracked. These kinds of focus errors or tracking errors occur regardless of whether the physical distortions such as the radial tilt or the tangential tilt, are compensated.
As a result of the above described fluctuations, the optical pickup Os can not make the laser spot focus on the aimed recording track or trace thereof. Specifically, the laser spot thus focused bridges two neighboring tracks, scans the recording tracks with varying angles, and scans a little or partial portion of the aimed recording track. Furthermore, the laser spot can not trace the recording track, and may loose the track of the aimed recording track. Therefore, the signal reproduction faces the problems of crosstalk, insufficient amplitude, noise, wave form interference, and jittering (asynchronization with the original signal), thereby causing the reproduced recording signal to jitter. When the jittering becomes greater, the quality of reproduced recording signal is considerably degraded. Furthermore the reading error of the optical pickup increases.
In FIG. 76, the relationship between the jittering amount (%) and the radial tilt (.theta.r), obtained by experimentation, is shown. The jittering amount shows its minimum value within a range from +0.1 to -0.1 of a degree with respect to the zero-radial tilting position. It is also apparent that the jittering increases at a greater rate as the radial tilt .theta.r becomes greater.
Therefore, in order to reproduce the original signal from the recording track under such fluctuating factors, it is necessary to adjust the optical disk apparatus with respect to the physical fluctuations such as radial tilt and tangential tilt, and the electrical fluctuation such as the focus errors and tracking errors.
With reference to FIG. 74, a conventional optical disk apparatus which compensates for the radial tilt is described. The conventional radial optical disk apparatus includes an optical disk 8101, an optical head 8102, a tilt sensor 8103, a preamplifier 8104, a radial tilt controller 8105, and a radial tilt adjuster 8106.
In operation of thus constructed conventional optical disk apparatus, the tilt sensor 8103 outputs a tilting signal Sr having a voltage corresponding to the tilting amount of the optical disk 8101 with respect to the optical head 8102. The preamplifier 8104 amplifies the tilting signal Sr. The radial tilt controller 8105 drives the radial tilt adjuster 8106 such that thus amplified tilting signal Sr becomes zero. Thus, the optical head 8102 can be kept in a position parallel to the optical disk 8101 so that a signal with a good quality can be recorded thereto or reproduced therefrom.
However, with the configuration mentioned above, the tilt sensor (8103) is an essential component, thus causing an expensive manufacturing cost, and the sensor should be arranged beside the optical head (8103), thereby preventing the miniaturization of the optical disk apparatus. Furthermore, the tilt sensor should be correctly located such that it is parallel to the optical head, otherwise the parallel nature of the optical head with respect to the optical disk can not be adjusted correctly.